Replace the "Install" / "Installed" button by a "Remove" button after installation

[nekohayo]

After installing an extension, the UI looks like this:

However, the "Installed" button is not interactive, and not terribly helpful. What would be expected instead is for this button to be replaced by a red "Uninstall" or "Remove" button.

It would probably also make sense to do that in the search results ("Browse") view too, though that's probably less critical.